Why didn't they join the rebel alliance during the dark times era/TFU time?

It is my understanding that Yoda went into exile because he couldn't destroy the emperor. He then went into exile on Dagobah. Then he encounters Starkiller. Yoda must realize that he is a very powerfull force sensitive and that he is not of the Sith ( I think it was pretty apparent that Starkiller was a not a Sith, because of his mellow nature in the cutscene). Why wouldn't he have said something? Has Yoda lost all hope in the Jedi or destroying the Sith? Surely Yoda and Starkiller could have destroyed the Emperor and Vader. He could have atleast tried to destroy them. Yoda then is reluctant to train Luke. Did Yoda not care about the future of the Jedi or destroying the Sith. He lets an extremely powerful force sensitive go without figuring out anything about him and he is reluctant to train the offspring of the person with the highest minichlorian count ever. What is Yoda doing?

Obi Wan goes into exile because he will watch over Luke. I don't understand why he didn't train Luke when he was young. If Luke was trained young, he could have easily destroyed the emperor and Vader. I have heard that the emperor would feel a new force user, but then why couldn't the emperor sense Obi Wan was where he was? Obi Wan definitely knew of the Rebel Alliance, because there were alot of people of Tattoine that were a part of it. Why didn't he join? Its almost as if he didn't care about the future of the Jedi or destruction of the Sith? Why didn't he want to destroy the sith?

Imagine if Kota, Starkiller, Obi Wan, and Yoda were part of the rebel alliance? Game over Emperor